Magnetic fields are widely applied in measurement and imaging technologies. For example, various types of magnetic resonance imaging (MRI) apparatus acquire molecular structures and structural information about the interior of human bodies, for example, by using nuclear magnetic resonance phenomena on the basis of the magnetic fields generated by magnets. In order to obtain MR images of high quality, a magnet in the MRI apparatus is to provide a magnetic field with very high homogeneity in a certain specific space (e.g., within a detection area). Although this may be easily achieved at the center position of the magnet, it is very difficult to provide the homogeneity of the magnetic field at eccentric positions of the magnet. For example, since a shoulder coil for scanning the shoulder of a human body may be deployed at a position close to an edge of a bore of the magnet, and the main magnetic field here may be inhomogeneous, the image quality of some specific images (e.g., images by fat suppression technology and images by water excitation technology) in these parts is significantly lowered.
A shimming device (e.g., a shim shell) may be placed in the magnet of the MRI apparatus, so as to compensate for the lack of homogeneity in the main magnetic field (e.g., B0 field). For example, a method for compensating an inhomogeneous magnetic field is proposed in Chinese patent application no. 200810239127.3. As another example, a local coil for magnetic resonance equipment is proposed in Chinese patent application no. 200710195529.3, with a second coil in the local coil carrying out a shimming process on the area where the local coil is located. The local coil is provided with relatively poor shimming effects in some areas (e.g., in an area that is non-homocentric with the main magnetic field). As another example, an improved shim for an imaging magnet is proposed in Chinese patent application no. 200810093332.3. The shimming device may be formed by some passive shimming pieces or shimming coils. Since the change of the main magnetic field at eccentric positions (e.g., edge positions) of the magnet is relatively fast, this makes the shimming effects of the shimming device quite sensitive to the deployment accuracy of the shimming device. In other words, better shimming effects may be provided only if the shimming device is deployed accurately at a position passing through the center of the cross section of the magnet.
In order to achieve the deployment of the shimming device, a laser mark is provided on the shimming device, and the shimming device with the laser mark is installed onto a test bed (e.g., a patient table (PTAB)). The laser mark on the shimming device is positioned using a laser emitted from the MRI apparatus, and the test bed with the shimming device is moved to a predetermined position according to the positioning point so as to locate the shimming device on the test bed at a center of the cross section of the magnet. By way of theoretical calculation, at this moment, the shimming device should be exactly located at the center position of the cross section of the magnet. However, in practical applications, due to engineering implementation and processing factors or changes in the environmental conditions during the transportation, installation and use of the MRI apparatus, it is very difficult to achieve the expected homogenous magnetic field distribution by way of simply positioning the shimming device using a laser. For this reason, the deployment point of the shimming device is to be adjusted, so as to find the optimum deployment point of the shimming device and to achieve better shimming effects.